1. Field of the Invention
The present invention relates to calibration of a quadrature receiver. Such a receiver operates in the so-called 2.4 GHz ISM band, for instance, or can be any other suitable receiver.
2. Description of the Related Art
U.S. Pat. No. 5,949,821 discloses a radio communication device that demodulates a signal using in-phase and quadrature components. The radio communication device has an equalizer for correcting phase and gain imbalances that would otherwise distort a received signal. The equalizer determines peak amplitudes for the in-phase and quadrature components of the received signal. The phase imbalance between the in-phase and quadrature components is determined using the peak amplitudes. The peak amplitudes and phase imbalance are used to adjust at least one of the in-phase and quadrature components to correct for any imbalance therebetween. A phase estimator estimates the phase imbalance between the in-phase and quadrature components that were separated from a sampled down-converted received signal by a pair of quadrature mixers by mixing with respective sin(xcfx89t) and cos(xcfx89t) signals. The phase imbalance is determined as the arcsin function of the product of two over the product of peak amplitude estimates of the uncorrected in-phase and quadrature components and the cross-correlation between the uncorrected in-phase and quadrature components. The equalizer thus corrects amplitude and phase imbalances of a received signal, i.e. a useful signal.
It is an object of the invention to provide a quadrature receiver with phase adjustment of quadrature components of a local oscillator, thereby using quadrature output signals that represent narrow band noise signals.
It is another object to provide a quadrature receiver wherein such phase adjustment is independent of gain errors in in-phase and quadrature branches of the receiver.
It is another object of the invention to provide a quadrature receiver wherein such phase adjustment uses a phase adjustment signal obtained from cross-correlated quadrature output signals of the receiver.
In accordance with the invention, a method of calibrating a receiver is provided, the method comprising:
ensuring that an input of said receiver receives no useful signals;
cross-correlating quadrature output signals provided by said receiver, said quadrature output signals representing noise signals; and
adjusting a relative phase of quadrature local oscillator signals that are used to generate said quadrature output signals through down-conversion of a received signal, said adjusted relative phase being obtained from said cross-correlated quadrature output signals.
The invention is based on the insight of generating balanced quadrature signals at an early point in the receiver, i.e. at down-conversion of the received signal so that no further complicated corrections are needed, and further on the insight that cross-correlation of low pass filtered noise signals provides a signal that represents a phase imbalance that is independent of gain errors in the in-phase and quadrature branches of the receiver.
In one embodiment, the receiver is a zero-IF receiver. In this embodiment the relative phase of the quadrature local oscillator is set at a minimum cross-correlation value. In this embodiment advantageously DC-offsets in the in-phase and quadrature branches are removed by AC-couplers.
In another embodiment, the receiver is a low-IF receiver. In this embodiment the relative phase of the quadrature local oscillator is set at a zero cross-correlation value.
In order to ensure that no useful signals are received during phase calibration, an input of the receiver may be decoupled from an antenna that receives useful signals. Herewith it is ensured that cross-correlation is purely based on noise signals.